Methods of separating and feeding stacked sheets, such as documents and recording sheets include frictional, vacuum, and electrostatic methods.
The frictional feeding method using frictional force uses, for example, a rubber material to form a feed roller. Changes in the properties of the feed roller over time due to abrasion, etc., may cause a change in frictional force and degrade feeding performance. Further, when sheets varying or not uniform in coefficient of friction or sheets having different coefficients of friction are separated and fed in the same feeding operation, a feeding failure, such as simultaneous multiple feeding of a plurality of sheets and a failure to separate sheets, may occur. Further, the method is based on a configuration that separates sheets by applying pressure thereto in the sheet feeding operation, and thus may stain the sheets.
The vacuum feeding method using air suction is a non-frictional separation method not relying on the coefficient of friction of rollers and sheets. The method, however, uses an air suction blower and an air duct. Thus, the sheet feeder according to the method is increased in size, and air suction sound causes noise. Therefore, the sheet feeder is not suitable for use in an office environment.
The electrostatic feeding method using electrostatic attraction includes a variety of sheet separation methods all involving manipulating a sheet attraction unit or a sheet loading unit.
In a conventional sheet feeder based on the electrostatic attraction separation method, an alternating voltage is applied to a surface of an endless dielectric belt wound around a plurality of rollers to form a charge pattern on the surface. Then, the endless belt is swung or translated relative to a stacked sheet bundle and brought into contact with or proximity to the sheet bundle to attract the uppermost sheet of the sheet bundle to the endless belt. Thereafter, the endless belt is moved in a direction separating from the sheet bundle to separate and feed the uppermost sheet from the sheet bundle. The method, which is a type of non-frictional separation method, is advantageous in preventing abrasion, sheet damage, and noise, and in allowing a reduction in size of the sheet feeder.
Most background sheet feeders using electrostatic attraction employ a method of lifting and lowering a sheet loader bottom plate with every operation of attracting and separating a sheet. In such background sheet feeders, an attraction device is basically stationary. In some of the background sheet feeders, however, the downstream side of the attraction device is lifted, or the upstream side of the attraction device is lowered to hang.
In recent years, sheet feeders employing a method of lifting and lowering the entire attraction device have become more common. According to this arrangement, the bottom plate is lifted to a predetermined height after a sheet feeding tray is set in the sheet feeder, or the bottom plate brought into contact with the attraction device is lowered to and stopped at the predetermined height. Thereafter, the attraction device is lifted and lowered to perform the operation of attracting and feeding each sheet. During the operation, the attraction device, specifically a belt unit, swings with the upstream side thereof in the sheet feeding direction hanging down, thereby turning a sheet from a sheet bundle.
According to the sheet feeders employing the above-described method using electrostatic attraction, in the separation of the endless belt from the stacked sheet bundle, the sheet separation performance is substantially affected by the distance of separation of the endless belt from the stacked sheet bundle according to the lifting of the endless belt and by the angle of the separated endless belt relative to the upper surface of the sheet bundle. An increase in the angle of the endless belt should improve the sheet separation performance in accordance with the rigidity or firmness of the sheet, even if the separation distance is unchanged. The increase in the angle of the endless belt, however, may cause the sheet attracted to the endless belt to separate from the endless belt owing to the resilience of the sheet. That is, the optical separation distance and angle of the endless belt varies depending on the type of sheet. Yet in the background sheet feeders, which simply translate or swing the endless belt, each of the separation distance and the angle of the endless belt is set to a fixed value. It is therefore difficult to appropriately adjust the characteristics of the separation distance and the angle of the endless belt.